1. Field of the Invention
This invention relates generally to medical diagnostic instruments, and specifically to an ophthalmoscope for use in retinal viewing.
2. Background of the Prior Art
Ophthalmoscopes for use in imaging a patient's retina include an optical imaging path for imaging a retina through a pupil and an illumination path for illumination of a retina by light rays entering an eye through a pupil.
A challenge inherent in designing such devices is that both the optical imaging path for imaging a retina and the optical illumination path for illuminating the retina are both ideally centered on the same axis, the optical axis of the pupil of an eye being viewed.
In general, there have been two approaches in response to this inherent challenge of ophthalmoscope design. A first approach, as in a traditional direct view ophthalmoscope, is to move the illumination pathway off axis slightly so that a retina can be directly viewed. Problems with this approach are that illumination light rays do not illuminate the same area being viewed, and that there is not appropriate space in an optical imaging path of such a system for a lens assembly which would expand a field of view.
A second approach to the inherent challenge mentioned above is to design an indirect ophthalmoscope which provides for larger field of view by disposing a partially transmitting illumination light source, such as a beam splitter, which allows receive optical light to pass therethrough, embodied for example in Model 11305 ophthalmoscope available from Reichert Ophthalmic. A major drawback with such a design is that the system is affected by significant light losses. An indirect ophthalmoscope embodying the beam splitter approach embodies a typical light loss of about 75% throughout the system. Because of the system light losses in such ophthalmoscopes, ophthalmoscopes embodying the beam splitter approach require high powered (e.g. above about 10 W) light sources in order to provide sufficient illumination of a retina. High power light sources, in general, are difficult to package, consume excessive amounts of input power, and produce large amounts of heat and unwanted light such as glare.
Cameras for retinal imaging, such as fundus cameras, provide high quality imaging, but are expensive, typically require pupil dilation for entry into an eye, and typically require operation control by a highly skilled and trained camera operator.
There is a need for a compact, lower power ophthalmoscope which provides sufficient retinal illumination and which facilitates wide field retinal imaging without requiring pupil dilation.